The blades of one piece turbine wheels in gas turbine engines are subjected to many excitation frequencies during turbine operation. These excitation frequencies are developed by the various components of the turbine engine as well as its accessories.
In attempting to damp the vibration in the turbine blades resulting from such excitation, several problems must be attended to. For one, many natural frequencies with various vibration mode shapes exist for the blades. The blades also operate in a high temperature environment over a broad range of high temperatures as, for example, 700.degree. F.-1800.degree. F.
Not infrequently, the material of which the turbine wheel is formed has low natural damping characteristics. Further, in one piece turbine wheel constructions, assembly of a damping device is difficult because of the geometrical configuration of the one piece construction.
Typically, the blades of the turbine wheel are very thin and high stresses will exist during operation. This will pose restrictions on the additional mass that may be allowed in or added to the blade for damping purposes.
Care must be taken so as not to restrict the flow of the hot gasses and good reliability and wear characteristics are required.
To overcome the foregoing difficulties, there have been a variety of proposals for mechanical dampers that may be applied to turbine wheels. See, for example, U.S Pat. Nos. 2,772,854 issued Dec. 4, 1956 to Anxionnaz; 3,291,446 issued Dec. 13, 1966 to Huebner; and 3,754,838 issued Aug. 28, 1973 to Fu. In addition, reference should be made to French Pat. No. 1,024,218 granted Jan. 7, 1953 to Societe Rateau on an invention by Anxionnaz, such French patent being published on Mar. 30, 1953.
The Anxionnaz and Fu patent publications illustrate damping of turbine blades while the Huebner patent illustrates provision of damping structure within part of the turbine wheel hub itself. In the case of Anxionnaz and Fu, various structures may be used including rivet-like elements that extend through the blade and which are double headed for retention purposes. The Anxionnaz French patent specifically shows such a device intended to achieve friction damping while Fu shows a somewhat similar device but modified so as to achieve impact damping.
In any event, while these structures do achieve their intended purpose, they are not altogether satisfactory because they do not provide a means whereby the mass of the damper can be selected and/or controlled generally independently of the size of the hole within the blade in which the mechanical damper is received.
More particularly, those skilled in the art will readily recognize that stress concentration around a hole formed in a turbine blade increases as a multiple of hole size. At the same time, the hole formed to receive the mechanical damper must be of sufficient size as to allow easy installation while the damper itself must be of a size so as to be retained within such hole. In some cases, this can result in the mass of the damper being larger than desired and that in turn could lead to unduly high stress level at the hole during operation of the turbine wheel in a gas turbine engine.
The present invention is directed to overcoming one or more of the above problems.